As a method for electrically bonding first and second bonding target members with high bonding strength, ring mash (trademark) bonding, for example, is known (see Patent Documents 1 and 2). This ring mash bonding method includes providing a first bonding target member having a bonding target portion and a second bonding target member having a hole portion with a bonding target portion having an inside diameter which is slightly smaller than the outside diameter of the bonding target portion of the first bonding target member, positioning and placing the first bonding target member on the second bonding target member such that the bonding target portion of the first bonding target member slightly overlaps the bonding target portion of the hole portion of the second bonding target member, and applying a current, in this state, to the first and second bonding target members under pressure until their bonding target portions undergo plastic flow and pressing the bonding target portion of the first bonding target member into the bonding target portion of the hole portion of the second bonding target member to solid-phase bond the first and second bonding target members.
A feature of this ring mash bonding is that because the bonding target portion of the first bonding target member and the bonding target portion of the second bonding target member increase their bonding areas while undergoing plastic flow by the effect of the pressing force and bonding current and a vicinity of an outer peripheral surface of the bonding target portion of the first bonding target member and a vicinity of an inner peripheral surface of the bonding target portion of the second bonding target member are eventually solid-phase bonded to each other with a certain bonding width, high bonding strength can be achieved without being significantly affected by the dirt on the surfaces or the roughness of the surfaces of the bonding target portions. The bonding strength by the ring mash bonding depends on the bonding area, and the bonding area is proportional to the bonding diameter×bonding depth.